Window glass typically serves as a medium for the transmission of large amounts of heat in a building. For example, the proportion of heat that escapes from windows during heating in winter reaches about 48%, while proportion of heat that enters from outside windows during cooling in summer reaches about 71%. Similar phenomena also apply to automobiles in which window glass also serves as a medium for the transmission of large amounts of heat. In automobiles, the proportion of window glass to space inside the vehicle is larger than that in buildings, and since there is little margin for persons in an automobile to avoid sunlight, the inside of an automobile subjected to an environment of intense sunlight reaches extremely high temperatures.
According to examples of measurements made in a summer environment in Japan, the air temperature inside a parked car reaches nearly about 70° C. With respect to the temperatures of interior components inside an automobile, that of the top of the instrument panel rises to nearly 100° C., while that of the ceiling rises to nearly 70° C. It goes without saying that a person inside such an automobile under these circumstances would be quite uncomfortable. In addition, the temperatures of interior components do not readily decrease even when opening the windows or using the air-conditioner, thereby continuously subjecting passengers to radiant heat over a long period of time and resulting in a considerable decrease in comfort within the vehicle.
Dimming glass has been developed that is capable of controlling the transmission of light and heat as a technology for solving these problems. There are several types of dimming methods used in dimming glass. Examples of dimming devices include: 1) electrochromic devices that use a material that undergoes a reversible change in transmittance by applying a current or voltage, 2) thermochromic devices that use a material that undergoes a reversible change in transmittance according to temperature, and 3) gas-chromic devices that use a material that undergoes a change in transmittance by controlling an atmospheric gas.
Among these, electrochromic devices are able to electrically control the transmission state of light and heat. Consequently, electrochromic devices are extremely suitable for use as dimming materials applied to building and automotive glass as a result of allowing the transmission state of light and heat to be set to an intended state. Moreover, since the optical characteristics of these devices do not change when a current or voltage is not applied thereto, the energy required to maintain a constant state can be reduced.
Although a portion of the constituents of electrochromic devices may be a liquid, in such cases, it is necessary to prevent leakage of the liquid. Although buildings and vehicles are premised on long-term use and it is possible to prevent leakage over a long period of time, this results in increased costs. Consequently, electrochromic devices suitable for building or vehicle glass is preferably that in which all of the materials that compose the device are in a solid state in the manner of tungsten oxide.
Tungsten oxide and other known electrochromic devices function based on the principle of dimming light by absorbing light with a dimming material. Namely, these devices inhibit heat in the form of light from entering the interior by absorbing light. However, in the case of employing a dimming material that uses this type of dimming principle, there is the problem of the dimming material retaining heat as a result of absorbing light, thereby causing that heat to again be radiated into the interior and resulting in heat penetrating into the dimming glass.
As means for solving this problem, a technique has been conceived in which dimming is carried out by reflecting light instead of by absorbing light. In other words, entry of heat into the interior caused by absorption of heat by a dimming material can be prevented by using a reflective dimming material that reversibly changes between a mirrored state and a transparent state.
An example of a reflective dimming electrochromic device having such properties disclosed in the prior art is an electrochromic device comprising the lamination of a reflective dimming layer composed of an alloy of a rare earth metal and magnesium and a hydride thereof, a proton-conducting, transparent oxidation protective layer, an anhydrous solid electrolyte layer and an ion storage layer (see Patent Document 1).
The reflective dimming layer has a function that controls reflectance of the electrochromic device, and changes reflectance by transferring protons. The oxidation protective layer is composed of a compound having proton conductivity in the manner of an oxide such as niobium oxide, vanadium oxide or tantalum oxide, or a fluoride such as magnesium fluoride or lead fluoride, and prevents oxidation of the reflective dimming layer.
The ion storage layer stores protons used to control reflectance. When a voltage is applied to dimming glass, protons migrate from the ion storage layer to the reflective dimming layer through the solid electrolyte and oxidation protective layer, and the reflectance of the reflective dimming layer changes. When an opposite voltage is applied, protons are released from the reflective dimming layer, and reflectance of the reflective dimming layer returns to its original state. In this device, however, since an expensive rare earth metal is used in the reflective dimming layer, it is difficult to apply this device to large areas from the viewpoint of cost.
An example of another reflective dimming device that uses an inexpensive and more practical material for the reflective dimming layer has been proposed in which Mg2Ni is laminated as the reflective dimming layer and palladium or platinum is laminated as a catalyst layer (see Patent Document 2). However, this material is completely unable to be used practically due to the low level of transmittance when transparent.
Although a magnesium-nickel alloy thin film developed by a portion of the inventors of the present invention (see Patent Document 3) is of the gas-chromic type that uses hydrogen gas, the visible light transmittance of this thin film is about 50%, which is a considerable improvement over the level of 20% of the previously reported Mg2Ni, bringing it closer to practical use. An example of an all-solid-state dimming mirror device using this magnesium-nickel alloy thin film has been proposed in the form of an all-solid-state dimming mirror light switch comprising the lamination of an ion storage layer, a solid electrolyte layer and a reflective dimming device in the form of the magnesium-nickel alloy described in Patent Document 3 on a transparent substrate (see Patent Document 4).
However, this device has problems with durability, and although it demonstrates switching durability of 1000 cycles or more, it has the shortcoming of not returning to a reflective state due to the occurrence of deterioration. One possible cause of this was suggested to be the gradual diffusion of reflective dimming layer components and catalyst layer components into the solid electrolyte layer accompanying repeated switching (see Non-Patent Document 1).
This shortcoming was the same in devices using a magnesium-titanium alloy thin film or magnesium-niobium alloy thin film. Consequently, there has been a strong desire in this technical field for the development of an all-solid-state reflective dimming electrochromic device having high transmittance when transparent, enabling switching over a wide area, and having high durability.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-204862
Patent Document 2: U.S. Pat. No. 6,647,166
Patent Document 3: Japanese Patent Application Laid-open No. 2003-335553
Patent Document 4: Japanese Patent Application Laid-open No. 2005-274630
Non-Patent Document 1: K. Tajima, Y. Yamada, O. Bao, M. Okada and K. Yoshimura, “Durability of All-Solid-State Switchable Mirror Based on Magnesium-Nickel Thin Film”, Electrochemical Solid-State Letters, Vol. 10, No. 3, pp. J52-54, 2007
With the foregoing in view, as a result of conducting extensive studies for the purpose of developing an electrochromic device capable of providing a radical solution to the problems described above, the inventors of the present invention succeeded in improving durability by using an aluminum thin film as a buffer layer for the purpose of preventing diffusion of constituents between all-solid-state reflective dimming electrochromic devices using a magnesium alloy thin film, thereby leading to completion of the present invention.